This invention relates to an automatic adjuster for spring applied mechanisms and has been devised particularly though not solely for use with spring applied fail-safe brakes.
There are many different brake and clutch mechanisms used in many different applications, such as the disc brakes used in motor vehicles and various industrial applications such as conveyor belts where it is necessary to automatically adjust the wear or lost travel in the piston mechanism. Such automatic adjusting mechanisms are readily available in use as the problem has been easily solved when the brake piston travel is in the same direction in which the adjustment needs to occur. In this configuration it is a simple matter to extend the adjusting mechanism to compensate for lost travel, and many such mechanisms are commonly available.
By way of contrast, so-called xe2x80x9cfail-safexe2x80x9d disc brakes use spring energy to supply the force to apply the brake. The brake is released by hydraulic, pneumatic or direct mechanical actuators. In fail-safe brakes of this type, if there is any failure in the release mechanism, the brake is automatically applied by the spring force.
One of the major difficulties in designing xe2x80x9cfail-safexe2x80x9d braking systems is the fact that the spring force is not linear in relation to travel. This requires constant adjustment to compensate for wear (of the brake pads) and keep the spring travel to a minimum. Alternatively some designs utilise a spring which is much stronger than is required in optimum conditions, so that the spring will supply the desired braking force even when the brake pads have been worn to their maximum extent.
The difficulty in providing an automatic adjuster for fail-safe brakes is that the direction of travel of the piston to operate the brake is in the opposite direction to that required for the automatic adjustment of lost travel. Because of this difficulty, at present all adjustments for lost travel are carried out by manual adjustment or by external ratchet-type adjusters. This is of course undesirable as it requires frequent attention to the braking mechanisms with the possibility that the adjustment could be overlooked and the fail-safe braking systems move outside the normal tolerances for efficient operation. Furthermore the necessity for frequent manual adjustment is time consuming and therefore expensive.
The present invention therefore provides an automatic adjuster for spring applied mechanisms of the type wherein travel of an actuating member under spring pressure may vary depending upon component wear, the adjuster incorporating a piston movable by spring pressure in one direction to apply a desired force and by other actuating means in the opposite direction to remove the desired force, a pushrod movable by and with the piston, and a lock actuator operable between the piston and the pushrod arranged to provide positive movement of the pushrod with the piston in the direction of force application from spring pressure while allowing relative movement between the piston and the pushrod over controlled limits during movement of the piston in the opposite direction.
Preferably the lock actuator comprises a collet operable between the piston and the pushrod.
Alternatively the lock actuator comprises an equivalent mechanism such as an offset collar and rod mechanism, self locking screw thread, or any other form of one-way locking device.
Preferably a further lock actuator is provided operable between the pushrod and a housing for the piston, arranged to control movement of the pushrod between predetermined limits during retraction of the pushrod.
Preferably the further lock actuator comprises a collet operable between the pushrod and the piston housing.
Alternatively the lock actuator comprises an equivalent mechanism such as an offset collar and rod mechanism, self-locking screw thread, or any other form of one-way locking device. Other methods of clamping having predetermined torque values, such as a compressed O-ring or various shrink rings can also be used.
Preferably the adjuster incorporates an extension of the piston operable to disengage said further lock actuator and allow the pushrod to be retracted to its original (unadjusted) position.
Preferably the adjuster incorporates a release cup operable from outside the adjuster to release the lock actuator from forming a clamp between the piston and the pushrod.
Preferably the actuating member is arranged to actuate the clamping motion of a pair of disc pads in a disc brake.
Preferably the release cup is operable to enable retraction of the pushrod for pad replacement in the disc brake.